Mould fluxes and their use in the continuous casting of steel

ABSTRACT

A granular mould flux for use in the continuous casting of steel comprises refractory metal oxide, one or more fluxing agents, a binder and 0.1% to 3% by weight based on the weight of the flux of an expanding agent such as expandable perlite, expandable vermiculite or expandable graphite.

This invention relates to mould fluxes and their use in the continuouscasting of steel.

In the continuous casting of steel a mould flux is generally added tothe surface of the molten steel in the mould. The flux provideslubrication between the mould wall and the steel, it reduces the loss ofheat from the surface of the steel, it protects the surface fromoxidation, and it may remove impurities such as alumina from the steel.

As granules evolve much less dust compared with powder, mould fluxesused in the continuous casting of steel are often used in the form ofgranules, which may be produced by, for example, spray-drying of theflux constituents. The excellent flowability of granules makes themparticularly suitable for automatic feeding to the mould, for example,using a DAPSOL (trade mark) feeder. However once the flux is in themould the flowability of the granules becomes a disadvantage since thegranules tend to find their own level under high rates of flow of steelinto the mould and the surface of the steel may become exposed in thecorners of the mould.

It has now been found that the above problem can be alleviated if thegranules contain a minor mount of an expandable material which willexpand under the action of heat and will cause the granules to breakdown into powder on the surface of the steel.

According to the invention there is provided a granular mould fluxcomprising refractory metal oxide, one or more fluxing agents, a binderand an expanding agent, the expanding agent being present in an amountof 0.1% to 3% by weight based on the weight of the flux.

According to a further feature of the invention there is provided amethod of continuously casting molten steel in a mould the methodcomprising adding to the mould prior to, during or after teeming of themolten steel a granular mould flux comprising one or more refractorymetal oxides, one or more fluxing agents, a binder and an expandingagent, the expanding agent being present in an amount of 0.1% to 3% byweight based on the weight of the flux.

The refractory metal oxide is preferably made up of calcium oxide andsilica but alumina and/or magnesia may also be present. Materials suchas blast furnace slag which contains calcium oxide, silica and alumina,or feldspar (sodium potassium aluminium silicate) which contains aluminaand silica may be used as a source of refractory metal oxides.

Wollastonite, which contains calcium oxide and silica, is a particularlyuseful component since it is capable of absorbing appreciable amounts ofalumina from the steel into the flux without significantly affecting theviscosity or melting point of the flux. The wollastonite component maybe, for example, a synthetic or natural calcium monosilicate (which maycontain very small quantities of iron oxide and/or alumina), or it maybe calcium monosilicate in solid solution with at least one of silica,calcium oxide or alumina, for example, a solid solution containingpseudo-wollastonite or rankinite.

The fluxing agent may be, for example, one or more of sodium carbonate,potassium carbonate, lithium carbonate, barium carbonate, sodiumfluoride, aluminium fluoride, potassium fluoride, cryolite, fluorspar,manganese dioxide and olivine. The fluxing agent reduces the meltingpoint of the flux and by the selection of particular fluxing agents andamounts the variation of the viscosity of the flux with temperature canbe controlled.

The binder may be any suitable binder which will maintain the integrityof the granules from manufacture through storage, transport and use upto the point of expansion of the expanding agent when it is necessaryfor the granules to disintegrate back into the original powder form.Examples of suitable binders include resins, gums such as apolysaccharide gum and carbohydrate materials such as molasses.

The expanding agent may be, for example, expandable perlite, expandablevermiculite or expandable, e.g. acid-heated, graphite. The expandingagent is preferably present in an amount of 0.3% to 1.5% by weight basedon the weight of the flux and is preferably expandable graphite.

The flux may also contain a light-weight refractory material such asexpanded perlite, expanded vermiculite, or pumice, to lower the overalldensity of the flux.

The flux may also contain a carbonaceous material, (in addition to anyexpandable graphite which may be present as the expanding agent), suchas charcoal, coke, anthracite, graphite or carbon black, to control themelting rate and sintering characteristics of the flux.

The flux will usually contain 45% to 90% refractory metal oxide, 10% to50% by weight of fluxing agent, 0.05% to 10% by weight of binder, 0% to10% by weight of light-weight refractory material, and 1% to 6% byweight of carbonaceous material other than expandable graphite.

The application rate of the mould flux to the mould will usually be inthe range of 0.3 kg/tonne to 1.1 kg/tonne of steel cast.

The granules may be produced by a method such as pan granulation butthey are preferably in the form of substantially spherical granulesproduced by spray drying an aqueous slurry of a mixture of the fluxconstituents. The granules are preferably in the size range of from 0.1mm to 1 mm in diameter.

As stated previously the granular mould flux of the invention breaksdown in contact with the steel in the mould producing a powder layer offlux on the surface and preventing exposure of the steel in the mouldcorners. Additionally the granular mould flux of the invention retainsthe advantages of known granular mould fluxes such as greaterhomogeneity compared with powder flux compositions, low dust productionand excellent flowability for ease of automatic application.

The following examples will serve to illustrate the invention:

EXAMPLE 1

Substantially spherical granules of size 0.1 mm to 0.8 mm diameter wereproduced by spray drying an aqueous slurry having the followingconstituents:

    ______________________________________                                                            % by weight                                               ______________________________________                                        Sodium carbonate      9.75                                                    Fluorspar             21.56                                                   Calcium silicate      37.99                                                   Expanded perlite      4.11                                                    Graphite              1.13                                                    Carbon black          1.23                                                    Manganese dioxide     7.70                                                    Sodium potassium aluminium silicate                                                                 10.78                                                   Barium carbonate      5.13                                                    Expandable graphite   0.52                                                    Polysaccharide gum    0.10                                                    ______________________________________                                    

The granules were added to a mould in which steel slab was continuouslycast at a temperature of 1520° C. at a rate of 0.6 kg/tonne. Thegranules readily broke down to form a complete powder cover on thesurface of the steel, and the slab produced was clean and defect free.

EXAMPLE 2

A granular mould flux (A) according to the invention was used incomparison with a granular mould flux (B) not according to theinvention. The compositions, by weight, of the two fluxes were asfollows:

    ______________________________________                                                        (A) % (B) %                                                   ______________________________________                                        Calcium silicate  50.9    50.7                                                Carbon black      3.3     3.3                                                 Sodium fluoride   9.8     9.8                                                 Calcium fluoride  7.8     7.8                                                 Olivine           5.9     5.9                                                 Feldspar          7.8     7.8                                                 Alumina           1.5     1.5                                                 Graphite          --      1.0                                                 Lithium carbonate 1.0     1.0                                                 Sodium carbonate  11.2    11.1                                                Polysaccharide gum                                                                              0.1     0.1                                                 Expandable graphite                                                                             0.7     --                                                  ______________________________________                                    

Flux (B) was in regular use on a continuous casting plant and under mostconditions provided excellent lubrication between the mould wall and thesteel. However, in exceptional circumstances when, due to flushing ofthe tundish nozzle, a rapid steel level rise took place in the mould,inadequate lubrication was provided, and sticking of the cast steel tothe mould sometimes occurred.

Modification of the flux composition as in flux (A), i.e. by replacingthe 1% by weight graphite with 0.7% by weight expandable graphite andmaking up the balance with an additional 0.2% by weight of calciumsilicate and 0.1% by weight of sodium carbonate gave an improvement inperformance in that sticking did not occur during rapid rises of thesteel in the mould. This improvement is believed to be attributable toflux (A) not running away so rapidly from the high spot and thus bettermaintaining the integrity of the lubricating layer of flux over thesteel.

We claim:
 1. A granular mould flux comprising:refractory metal oxide; abinder; a fluxing agent comprising one or more of sodium carbonate,potassium carbonate, lithium carbonate, barium carbonate, sodiumfluoride, aluminium fluoride, potassium fluoride, cryolite, fluorspar,managanese dioxide, and olivine; and
 0. 1-3% by weight based on theweight of the mould flux of an expanding agent;said granular flux beingin the form of spherical granules produced by spray drying an aqueousslurry of a mixture of the flux constituents, and in the size range of0.1 mm to 1 mm in diameter.
 2. A granular mould flux as recited in claim1 wherein said expandable graphite is present in an amount of 0.3-1.5%by weight.
 3. A granular mould flux as recited in claim 1 furthercomprising a carbonaceous material other than expandable graphite.
 4. Agranular mould flux as recited in claim 3 wherein said fluxcontains:45-90% by weight refractory metal oxide; 10-50% by weight offluxing agent; 0.05-10% by weight binder; 0-10% by weight of lightweight refractory material; and 1-6% by weight of carbonaceous materialother than expandable graphite.
 5. A granular mould flux as recited inclaim 1 wherein said flux contains:45-90% by weight refractory metaloxide; 10-50% by weight of fluxing agent; 0.05-10% by weight binder;0-10% by weight of light weight refractory material; and 1-6% by weightof carbonaceous material other than expandable graphite.
 6. A granularmould flux as recited in claim 5 further comprising more than zero lightweight refractory material.
 7. A granular mould flux as recited in claim1 wherein said binder is a resin, a gum, or a carbohydrate material. 8.A granular mould flux as recited in claim 7 further comprising acarbonaceous material other than expandable graphite.
 9. A granularmould flux as recited in claim 8 wherein said flux contains:45-90% byweight refractory metal oxide; 10-50% by weight of fluxing agent;0.05-10% by weight binder; 0-10% by weight of light weight refractorymaterial; and 1-6% by weight of carbonaceous material other thanexpandable graphite.
 10. A granular mould flux as recited in claim 9further comprising more than zero light weight refractory material.